The prepress operations associated with lithographic printing plate production are demanding and becoming increasingly complex. Depending on the demands and preferences of the customer, printers may be required to use both negative and positive original transparencies as the input materials to produce a finished plate. Additionally, the high quality of the finished plate must be maintained regardless of the type of original input and must be commensurate with the need to produce copies that may number in the millions. However, as a consequence of these increasing requirements, both unipolar and bipolar plates are now available to the printing industry so that a single negative or positive original (unipolar) or both negative and positive originals (bipolar) can be used as input for printing plate image production. Known unipolar plates comprise generally a single photosensitive layer on a hydrophilic substrate. Known bipolar plates comprise generally at least two photosensitive layers on a hydrophilic substrate and are known in the art as bilevel plates, referring to the two photosensitive layers.
Currently, if the printer uses a conventional unipolar plate such as a negative working or a positive working plate to reproduce a negative and a positive original, i.e., bipolar reproduction, one of the original input materials must be converted to suit the final plate mode. This is expensive and time consuming. If the printer resorts to the use of a bipolar plate as available in the art heretofore for reproduction of images from negative and positive originals, additional process steps in development, masking or exposure are invariably required which renders this alternative expensive and time consuming as well. The additional steps required in the present art of bipolar printing plate utilization flow from the fact that in order to obtain the requisite positive images on the final printing plate from a negative and a positive original input, a reversal of one of the original images needs to be accomplished during the photographic image transfer process.
Artisans in the photographic arts have found the problem of simplifying the production and lowering the cost of a lithographic printing plate produced from negative and positive originals to be a substantial challenge. A major objective of the instant invention is to effectively meet that challenge.
U.S. Pat. Nos. 4,576,901 and 4,581,321 disclose a process for producing negative copies by imagewise exposing a coating containing amide substituted 1,2-quinone diazides followed by heating, re-exposing and developing. The process permits the production of negative copies with the aid of a material which yields positive copies when it is processed in a conventional manner.
U.S. Pat. No. 4,927,741 discloses a process and apparatus for reversal processing to produce a negative of an original image on the plate. The printing plates is required to be exposed overall after heating. Exposure is carried out while the plate is conveyed through water.
U.S. Pat. No. 4,299,912 discloses a process for producing a lithographic printing plate using a plate containing two light sensitive layers, one of which is a gelatin-silver halide emulsion and the other is a nonsilver halide layer. The patent teaches the use of a proteolytic enzyme to aid in the removal of the gelatino layer after imagewise exposure followed by exposure of the non-silver halide layer.
U.S. Pat. No. 4,341,856 and 4,283,478 teach printing plates containing at least two light sensitive layers. Production of the final image containing printing plate entails at least two exposure steps.
U.S. Pat. Nos. 4,350,753 and 4,348,471 incorporated by reference teach positive acting printing plates having high pre-development image visibility, including positive coatings consisting of phenolic-Novolak resins. U.S. Pat. No. 3,635,709 incorporated herein by reference, teaches a positive acting lithographic printing plate wherein the light sensitive layer is a composition comprising an ester of 2-diazo-l-naphthol-4 (or-5)-sulfonic acid with a polyhydroxy phenol that is a condensation product of acetone and pyrogallol.
U.S. Pat. No. 3,567,445 incorporated by reference teaches a presensitized lithographic printing plate with two differentially spectrally sensitive layers separated by a Novolak resin. The base layer is of the type used in a negative-working presensitized lithographic plate while the top layer is a silver halide emulsion layer.
It is an object of the present invention to provide a process for the production of a lithographic printing plate containing printable images produced from negative and positive originals sequentially exposed before development of the plate.
Another object of the present invention is to provide a lithographic printing plate precursor having coatings sensitive to light of different exposure speed photographically processable according to the aforestated process.
As employed herein, the term printing plate precursor refers to a hydrophilic substrate having one or more light sensitive coatings or layers. The precursor plate is used to produce a lithographic printing plate useful in printing operations.